A Multi‐Model Ensemble Reveals a Diverging Hydrological Cycle in the Upper and Lower Transboundary Brahmaputra River Basin

ABSTRACT

The transboundary Brahmaputra basin is a significant component of the Asian Water Tower, housing approximately 66 million people in four countries and experiencing intense floods. However, this data‐scarce river has been subject to very few studies related to the long‐term assessment of its water budget. In this study, a multi‐model ensemble Indian Land Data Assimilation System (ILDAS) is set up to study the trends and variability in the water budget components from 1981 to 2020. The results indicate no significant long‐term trend in the annual basin‐averaged precipitation, evapotranspiration (ET), and runoff; however, spatial divergence in the hydrological cycle within the basin is observed, with the upper/Himalayan part showing an accelerating hydrologic cycle, contrasted by a decelerating hydrological cycle in the lower/plain part, which gets 70% of the basin rainfall. Our findings show that the winter precipitation in the basin has decreased by 5 mm per decade. The decreasing winter precipitation is confined to the lower part of the basin, which translates into a decrease in winter ET and runoff, whereas the upper part of the basin experienced an increase in precipitation in the pre‐monsoon and monsoon periods, resulting in increasing ET and runoff. It is observed that increasing runoff enhances streamflow in the upper basin; however, this signal is nullified by declining runoff towards the downstream region, leading to no significant trend in streamflow in the lower part of the basin. Further, it is observed that the TWSA variability in the basin is significantly dependent on groundwater storage variability and the amount of water stored in rivers and floodplains. These insights enhance our understanding of the complex variability of the hydrology of the Brahmaputra River basin.